Short answer: A whole lot less than the punditocracy would like you to believe.
Long answer: Oh, my goodness, people. Could we all just stop with the wild oscillations from day to day? No one in their right mind believed the polls showing Obama up by double digits in a single week. Clinton didn’t score a “comeback” because she didn’t go away: She has been strong in NH for the past year. Until a couple of weeks ago, Clinton had a 20 point edge on Obama and yet she won the state by only 2 points. How is that a comeback?
But what about all those polls showing the “Obama bounce”? They were ludicrous to believe. It takes time for actual views to shift, but polls are sensitive to short-period trends and, especially, news coverage. All of a sudden, after winning Iowoa, Obama was everywhere on the news. Of course his numbers ticked up.
Did Clinton “save” her candidacy by shedding a tear or by performing well at the debate or by saber-rattling? No, she “saved” it by having a strong organization in a friendly state. For five days, her supporters have been yelling at us that no one should count her out — correctly. But after castigating everyone for misinterpreting Iowa, they are exultantly misinterpreting New Hampshire. Excluding the post-Iowa media insanity, things turned out about as would be expected — indeed, I think Obama did better than anyone would have said just four weeks ago. (I mourn that my guy Edwards did not stage the upset that would truly have been an upset.)
Anyone with technical experience knows you don’t trust the gauge until it’s had time to settle down. Trying to spin Obama’s bounce into a steamroller mandate was silly. Trying to spin Clinton’s just-barely-held victory in a state she’d expected to win handily into a comeback of epic proportions… well, that’s just mendacious.